1. Field of the Invention
The invention relates to a developing apparatus for use in an image forming apparatus such as a laser printer, a copying machine or a facsimile apparatus utilizing an electrophotographic printing method or an electrostatic recording method.
2. Description of Related Art
In an image forming apparatus utilizing, for example, an electrophotographic printing method, the surface of an electrophotographic photosensitive member (photosensitive member), which is an image bearing member, is charged by charging means, whereafter light is applied to the surface of this photosensitive member, whereby an electrostatic image (latent image) is formed on the surface of the photosensitive member. This electrostatic image is developed as a toner image with the toner of a developer supplied from a developing apparatus. This toner image is transferred from the photosensitive member to a transfer material (such as recording paper, an OHP sheet or cloth) by transferring means, and thereafter is subjected to a fixing process by fixing means.
A developing method for making the electrostatic image into a toner image is classified broadly into mono-component development and dual-component development. In mono-component development, a mono-component developer substantially comprising only resin toner particles (toner) is used as a developer. On the other hand, in the dual-component development, use is made of a dual-component developer chiefly provided with resin toner particles (toner) and magnetic carrier particles (carrier). In both of the mono-component developer and the dual-component developer, an extraneous additive (auxiliary particles) is sometimes added to the toner particles for the stabilization of the charge ability of the toner and the adjustment of the fluidity of the toner.
As the mono-component development, the following dry type mono-component contact developing method has been proposed and put into practical use. This method uses a nonmagnetic mono-component developer as the developer. A developer carrying member carrying the developer thereon and conveying it to a photosensitive member is urged against or brought into contact with the rotating photosensitive member to thereby develop the electrostatic image. There is also a non-contact developing method of effecting development with the developer carrying member being out of contact with the photosensitive member.
The developing method using the nonmagnetic mono-component developer does not require a magnetic material for the developer, and facilitates the simplification and downsizing of an apparatus. Also, the developing method using the nonmagnetic mono-component developer is good in color taste and therefore has many advantages including the ease of application to a full-color image forming apparatus.
In a developing apparatus utilizing the nonmagnetic mono-component developer, the toner cannot be supplied to the developer carrying member by a magnetic force and therefore, other means for supplying the toner to the developer carrying member is required.
In the developing apparatus using such a nonmagnetic mono-component developer, a method using a supplying roller as a developer supplying member, which does not contact the developer carrying member has been proposed as means for supplying a toner to the developer carrying member (see Japanese Patent Application Laid-open No. H3-48877 and Japanese Patent Application Laid-open No. H11-212365).
That is, as shown in FIG. 11 of the accompanying drawings, such a developing apparatus 201 has a rotated developer carrying member 204, a seal member 211 contacting the developer carrying member 204, a supplying roller 205 opposed to the developer carrying member 204 with a predetermined interval therebetween downstream of the seal member 211 with respect to the movement direction of the surface of the developer carrying member for supplying a toner to the developer carrying member 204 while contacting with the toner, a holding member 212 for supporting the seal member 211, and a scraping member 213 supported by the holding member 212 for scraping off the toner held on the outer peripheral portion of the supplying roller 205. A space (containing portion) surrounded by the developer carrying member 204, the seal member 211, the holding member 212, the scraping member 213 and the supplying roller 205 forms a toner supplying portion 214 for supplying the toner contained therein onto the developer carrying member 204. As the developer carrying member 204, use has heretofore been made of a belt sleeve or a metal sleeve or an elastic roller or the like having a circumferential length slightly greater than the circumferential length of the rotary member and mounted around the rotary member.
A method of supplying the toner to the developer carrying member 204 in the developing apparatus 201 of such a construction is as follows. The toner held on the outer peripheral portion of the supplying roller 205 is conveyed on the basis of the rotation of the supplying roller 205, is scraped off by the scraping member 213 and is held in the toner supplying portion 214. Also, the amount of toner t contained in the toner supplying portion 214 is increased by the rotation of the supplying roller 205 with a result that toner pressure (powder pressure) there is kept high. By this high toner pressure, a necessary amount of toner is supplied to the developer carrying member 204 contacting the toner contained in the toner supplying portion 214.
Any residual toner on the developer carrying member 204 after the termination of the developing step (namely, after the passage through the contact portion with or the opposed portion to the photosensitive member) passes through the gap between the developer carrying member 204 and the seal member 211, and is returned to the toner supplying portion 214.
Here, when the toner newly supplied onto the developer carrying member 204 and the residual toner on the developer carrying member 204 after the developing step are mixed together, there occurs an image fault. So, heretofore, the seal member 211 is formed by an electrically conductive elastic member of sponge structure, such as electrically conductive moltopren, to thereby constitute a charge eliminating member, or a charge eliminating sheet, such as electrically conductive Teflon sheet, is been stuck on the upper portion of the seal member 211 to thereby constitute a charge eliminating member. A bias is applied to this charge eliminating member to thereby effect the charge elimination of the residual toner on the developer carrying member 204. Thereby, after the developing step, the toner is stripped off from the developer carrying member, whereafter a layer of new toner is formed on the developer carrying member 204.
In the conventional developing apparatus 201 as described above, however, even the residual toner on the developer carrying member 204 after the termination of the developing step, even if it passes the seal member 212 and thereafter is stripped off from the developer carrying member 204, is returned to the toner supplying portion 214 without separating from the developer carrying member 204. Thus, this toner returned to the toner supplying portion 214 is immediately supplied again onto the developer carrying member 204 and is used for development.
Now, the toner on the developer carrying member 204 is subjected to frictional contact with a developer regulating member 206, such as an elastic blade, and the photosensitive member. Therefore, if as described above, the toner returned to the toner supplying portion 214 is immediately supplied again onto the developer carrying member 204 and is used for development, when the apparatus is continuously used, heat and damage due to the frictional contact are accumulated in the toner. Then, the extraneous additive adhering to the outer peripheral portion of the toner is embedded into the toner or the extraneous additive separates from the toner, thus causing so-called toner deterioration. Accordingly, when the apparatus is continuously used, the fusion of the toner occurs to the developer regulating member 206. As the result, a streak-shaped irregular portion of toner occurs on the developer carrying member 204, and a streak-shaped density difference sometimes appear in an image.
Also, when as described above, a bias is applied to the charge eliminating member to strip off the residual toner on the developer carrying member 204 from the developer carrying member 204, the separation of the extraneous additive from the toner is liable to occur due to the electrical interference. Therefore, when the toner is continuously used, a reduction in the triboelectricity of the toner (the charge amount per unit weight of the toner) and the degree of aggregation increase, and in the image formation operation during the next round of the rotation of the developer carrying member 204, the density of the image is sometimes changed to thereby cause an image defect, such as a ghost image.
Thus, in the developing apparatus in which the toner is conveyed to the toner supplying portion 214 by the supplying roller 205, and the toner is supplied to the developer carrying member 204 with high toner pressure, it has heretofore been required to suppress an image defect such as a streak or a ghost caused by the residual toner on the developer carrying member 204 being immediately used for development after the passage through a developing area.
Also, to obtain the desired image density, there has been no way but to increase the supplying ability of the supplying roller to thereby supply more toner to the toner supplying portion. If the toner is not sufficiently supplied to the developer carrying member, imaging problems, such as a reduction in image density, a blank area and an uneven density, will occur.
For example, to increase the supplying capability for supplying the toner to the toner supplying portion, there has been proposed a method of making the surface movement speed (peripheral speed) of the supplying roller higher than the peripheral speed of a developing roller as the developer carrying member. To make the peripheral speed of the supplying roller higher than the peripheral speed of the developing roller, there is a method of making the outer diameter of the supplying roller larger than that of the developing roller to thereby make the number of revolutions of the supplying roller equal to the number of revolutions of the developing roller. Also, when the outer diameter of the supplying roller is equal to or less than the outer diameter of the developing roller, to make the peripheral speed of the supplying roller higher than the peripheral speed of the developing roller, the number of revolutions of the supplying roller must be made greater than the number of revolutions of the developing roller.
However, increasing the outer diameter of the supplying roller makes it difficult to downsize the developing apparatus, which has become important in recent years, and this is not preferable. Particularly, when the desire to produce a color image is taken into account, there is required a developing apparatus of four colors, i.e. yellow, cyan, magenta and black and therefore, the entire image forming apparatus is liable to become bulky.
Also, when the outer diameter of the supplying roller is made small and the number of revolutions thereof is made great, the mechanical load increases and thus, the driving portion of the apparatus becomes bulky from the necessity of performing high-speed rotation.
Further, when the printing speed is made high, the peripheral speed of the developing roller also becomes high and therefore, there is also a limitation in the peripheral speed of the supplying roller.
As described above, in the conventional developing apparatus, the supplying capability of the supplying roller must be increased in order to obtain the desired density, and such a goal has been unsuitable for downsizing the apparatus and achieving a higher speed operation.